Scotland Yard reopened its investigation into phone hacking today – four years after the only convictions in the case – after the News of the World passed on "significant new information" alleged to implicate one of the paper's top executives in the practice.

Shortly afterwards the paper announced that it had sacked its assistant editor (news), Ian Edmondson. This came hard on the heels of the arrival in London of its proprietor, Rupert Murdoch, said to be in town to deal with both the phone-hacking scandal that has engulfed the paper and his corporation's bid to take complete control of BSkyB.

Until shortly before Christmas the paper had always alleged that only one rogue reporter and a private investigator were involved in the practice, and the police had repeatedly insisted that there was no evidence available to link any other News Corporation employees with hacking.

Tonight a source close to the new police investigation said the latest evidence passed to the Metropolitan police so far amounted to only a small number of emails, although detectives believe there may be many more.

"It's hard to believe these are the only ones. There may be a shedload of shit still to come," said one source.

Part of the fresh police inquiry will look at whether this new evidence should have been uncovered by the original investigation, undertaken by the Met's former assistant commissioner Andy Hayman. Some officers are understood to feel that Hayman's team did not investigate sufficiently thoroughly at the time.

Tonight a senior Tory launched a strong attack on the police for failing to carry out a proper investigation of the phone-hacking allegations the first time around.

John Whittingdale, chairman of the Commons culture select committee – which last year accused News International of "collective amnesia" over the allegations – expressed astonishment that the Yard had finally decided to act, given the evidence in a range of documents in its possession for the last five years.

"I find it utterly extraordinary that the police have been sitting on these documents for five years and did absolutely nothing about them," Whittingdale said.

"I think they generally accepted News International's excuse that there was just one rogue reporter. But the police were sitting on documents that implicate Ian Edmondson and they did not question him."

Whittingdale said today's dramatic developments also raised questions for Andy Coulson, who announced his resignation as the Downing Street communications director last week, and Rebekah Brooks, the News International chief executive.

The announcement by the Yard came just a few hours after David Cameron endorsed the director of public prosecutions' separate decision earlier this week to widen inquiries.

The prime minister told MPs: "Let me be absolutely clear: phone hacking is wrong and illegal, and it is quite right that the director of public prosecutions is reviewing all the evidence, which should be followed wherever it leads.

"I do not think it is necessarily fair to say the police have not been active – after all, there have been prosecutions, convictions, and indeed imprisonments – but the law is quite clear and the prosecuting authorities should follow it wherever it leads."

News International indicated that it had acted after an internal investigation, which involved a trawl through Edmondson's emails, allegedly revealed correspondence relating to hacking.

A source close to the investigation said the emails could be interpreted as showing Edmondson was aware of phone hacking and came in evidence from Burton Copeland, a firm of solicitors hired to carry out the investigation, on Monday morning.

The new police investigation will be led by deputy assistant commissioner Sue Akers, who is a specialist in tackling organised criminal gangs. Scotland Yard has decided to transfer the investigation from the specialist operations division, led by John Yates, to the specialist crime directorate.

Yard insiders say Yates's replacement is not a sign of his having failed. Rather, he is acting as deputy commissioner and heads the Yard's counter-terrorism operation at a time when the threat level is severe – roles more deserving of his time than chasing a tabloid over allegations of phone hacking.

The police and phone hacking

9 July, 2009

Scotland Yard refuses to re-open hacking inquiry after reviewing Guardian allegations. John Yates, who has just taken over the inquiry, says: 'No additional evidence has come to light.'

12 November, 2010

'The Metropolitan police service has today provided a file to the Crown Prosecution Service relating to new material in connection with phone hacking [at the NoW].'

CPS will conduct 'a comprehensive assessment of all material in possession of the Met Police relating to phone hacking, following developments in the civil courts'.

26 January 2011

'The Met has today received significant new information relating to allegations of phone hacking at the News of the World in 2005/06. As a result, the Met is launching a new investigation to consider this material.'